In a typical thermal printer, a web-type dye-carrier containing a series of spaced frames of different colored heat transferable dyes is spooled on a carrier supply spool. The carrier is paid out from the supply spool and rewound on a take up spool. The carrier moves through a nip formed between a thermal print head having heaters and a dye-absorbing receiver sheet. The receiver sheet is clamped to a rotatable drum. The receiver sheet may, for example, be coated paper and the print head is formed of a plurality of heating elements. When heat is supplied to the dye-carrier by heaters built into a thermal print head, dye is transferred to the receiver sheet.
The density, or darkness, of the printed dye is a function of the temperature of the heater and the time the carrier is heated, in other words the energy delivered from the heater to the carrier.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heater, yielding a variable dye density image pixel on the receiver.
Digital image processing of the electrical signals is often used to improve the contrast of continuous tone prints.
A typical print head includes a ceramic substrate which is secured to a heat sink often made of aluminum.
Most manufacturers attach the ceramic directly to the aluminum heat sink. Since the aluminum and ceramic are not optically polished surfaces, the two surfaces will tend to have a large amount of void space. The void space is filled with air which acts as an insulator. The air filled void space makes the thermal resistance between the ceramic and the aluminum rather high. The high thermal resistance between the ceramic and aluminum causes localized heat build-up in the ceramic, resulting in a contrast loss in the printed image.
Some print head manufacturers use a thin strip of compliant material between the ceramic and the aluminum heat sink. This material is used in the electronics industry to provide better thermal conductivity between two surfaces (typically a power semiconductor and a heat sink). There are many types of material available, one type being polyimide plastic tape.
The purpose of the heat sink has been to solve the problem of excessive heat. Without proper heat dissipation, excessive temperatures can cause dimensional variations and effect the operation of electrical components. More specifically, the extreme temperatures produced during printing can damage the heaters and/or the shift register/driver integrated circuits mounted on the print head. If the thermal resistance between the ceramic and the aluminum were too low, there would be a significant reduction in printing efficiency.
Although the polyimide provides better thermal conductivity between the ceramic and the heat sink when compared to a bare joint, the polyimide does exhibit some thermal resistance.
Print heads utilizing polyimide exhibit a warm-up phenomena at the leading edge of a print. As the average temperature of the ceramic increases, the amount of dye transferred for a fixed electrical input increases. Effectively, this produces a density "ramp" in the dye transferred as the print head moves across the media. Conventional solutions to this problem include prewarming the print head before beginning the actual image transfer or modifying the data to correct for the initial lower ceramic temperature. A thermistor mounted in the head provides a means to sense the average temperature of the ceramic.